AppStoreから最新バージョンを検索してユーザーに通知するメソッドを作ってみました。
ユーザーに使ってもらうアプリを常に最新版にして欲しい場合などに、使えるかと思います。

UpdateChecker.h

#import <Foundation/Foundation.h>

@interface UpdateChecker : NSObject

+ (void)checkApplicationNewVersionWithAppStoreId:(NSString *)storeId foundBlock:(void (^)(NSURL *url, NSString *version))block;

@end

UpdateChecker.m

#import "UpdateChecker.h"

#define kAppStoreAPIURL @"http://ax.itunes.apple.com/WebObjects/MZStoreServices.woa/wa/wsLookup?id=%@&country=jp"
#define kAppStoreLink @"https://itunes.apple.com/app/id%@"

@implementation UpdateChecker

+ (void)checkApplicationNewVersionWithAppStoreId:(NSString *)storeId foundBlock:(void (^)(NSURL *, NSString *))block
{
    NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:kAppStoreAPIURL, storeId]];

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];

    [NSURLConnection sendAsynchronousRequest:request
                                       queue:queue
                           completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
                               if ([data length] > 0 && !connectionError) {
                                   dispatch_async(dispatch_get_main_queue(), ^{
                                       NSDictionary *appData = [NSJSONSerialization JSONObjectWithData:data
                                                                                               options:NSJSONReadingAllowFragments
                                                                                                 error:nil];
                                       NSArray *versionsInAppStore = [appData valueForKeyPath:@"results.version"];
                                       if ([versionsInAppStore count]) {
                                           NSString *appStoreVersion = versionsInAppStore[0];
                                           NSString *currentVersion = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"CFBundleShortVersionString"];
                                           if ([currentVersion compare:appStoreVersion options:NSNumericSearch] == NSOrderedAscending) {
                                               //最新バージョンが見つかった
                                               if (block) {
                                                   NSURL *storeURL = [NSURL URLWithString:[NSString stringWithFormat:kAppStoreLink, storeId]];
                                                   block(storeURL, appStoreVersion);
                                               }
                                           } else {
                                               //最新バージョンが見つからなかった
                                               return;
                                           }
                                       }
                                   });
                               }
                           }];
}

@end